1. Field of the Invention
The present invention relates to an imaging optical instrument for imaging light from an object on a recording surface.
2. Description of the Related Art
Generally, such an imaging optical instrument includes an aperture stop disposed in an optical system for limiting beams contributing to image formation, in order to secure excellent imaging performance or to secure a required focal depth. Such an aperture stop is formed of a light-shielding sheet of metal or plastic defining a circular or polygonal aperture. The aperture stop is fixed inside a sealed space defined by lenses and a barrel holding the lenses.
Where, for example, high power lasers are used as light sources to emit light for forming images on a recording surface, light of high energy level enters the optical instrument from the objects. In such a case, the aperture stop is heated to a high temperature as a result of shielding the high energy light.
The heat of the aperture stop reaches the lens barrel by thermal conduction, and may deform the lens barrel locally. Such a deformation disrupts a proper positional relationship between the lenses to lower imaging performance. The heat of the aperture stop filling the lens barrel interior by thermal convection heats up and expands the entire lens barrel. This results in an imaging position shifting along the optical axis. With a further increase in the temperature of the aperture stop, the aperture stop itself may melt or vaporize to lose its light-shielding function. The shielding material may adhere to lens surfaces to lower optical efficiency also.
It is conceivable to deal with this problem by arranging for the aperture stop to be held independently of the lens barrel. Where such a construction is employed, surfaces of the lenses arranged forwardly and rearwardly the aperture stop are exposed to atmosphere outside the lens barrel. Consequently, dust and the like present in the atmosphere could adhere to the lens surfaces to lower optical efficiency.